U.S. patent number 5,461,684 [Application Number 08/297,029] was granted by the patent office on 1995-10-24 for y-branch digital optical switch.
This patent grant is currently assigned to Alcatel N.V.. Invention is credited to Monique Renaud, Jean-Francois Vinchant.
United States Patent |
5,461,684 |
Vinchant , et al. |
October 24, 1995 |
Y-branch digital optical switch
Abstract
In a digital optical switch an input waveguide and two divergent
output waveguides constitute a guide structure. A median gap
between the two output waveguides constitutes a guide gap.
Electrodes control the refractive indices of the two output
waveguides to couple the input waveguide and one or both output
waveguides depending on the value of a control signal. The width of
the guide gap is increased in input and output transition areas to
render the variation of this width more progressive therein. This
is achieved by means of a median aperture at the end of the input
waveguide and a progressive variation in the inclination of the
facing edges of the output waveguides.
Inventors: |
Vinchant; Jean-Francois
(Bruyeres le Chatel, FR), Renaud; Monique (Saint
Cheron, FR) |
Assignee: |
Alcatel N.V. (Amsterdam,
NL)
|
Family
ID: |
9450452 |
Appl.
No.: |
08/297,029 |
Filed: |
August 29, 1994 |
Foreign Application Priority Data
|
|
|
|
|
Aug 30, 1993 [FR] |
|
|
93 10 367 |
|
Current U.S.
Class: |
385/22 |
Current CPC
Class: |
G02B
6/2804 (20130101); G02F 1/3137 (20130101); G02F
1/3138 (20130101) |
Current International
Class: |
G02F
1/29 (20060101); G02F 1/313 (20060101); G02B
6/28 (20060101); G02B 006/00 (); G02B 006/36 () |
Field of
Search: |
;385/20-26 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
J F. Vinchant et al, "Low Driving Voltage or Current Digital
Opitcal Switch on InP for Multiwavelength System Applications",
Electronics Letters, vol. 28, No. 12, Jun. 1992, pp. 1135-1137.
.
J. A. Cavailles et al, "First Digital Optical Switch Based on
InP/GaInAsP Double Heterostructure Waveguides", Electronics
Letters, vol. 27, No. 9, Apr. 1991, pp. 699-700. .
H. Okayama et al, "Reduction of Voltage-Length Product for Y-Branch
Digital Optical Switch", Journal of Lightwave Technology, vol. 11,
No. 2, Feb. 1993, pp. 379-387. .
French Search Report FR 489114..
|
Primary Examiner: Ullah; Akm E.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Claims
There is claimed:
1. Y-branch digital optical switch including a wafer at least in
part made from a transparent material having a refractive index
sensitive to electrical action, said wafer having:
--a guide plane and, in said plane:
--a rear edge,
--a front edge,
--an axis with abscissae increasing from said rear edge to said
front edge in a longitudinal direction defined by said axis, areas
being each defined by one said abscissa or between two said
abscissae,
--a righthand side,
--a lefthhand side, and
--a transverse direction joining said righthand and lefthand
sides,
said switch including:
--an input situated on said rear edge to receive a light wave,
--two outputs situated on said front edge to output said light
wave, said two outputs constituting a righthand output on the
righthand side of said axis and a lefthand output on the lefthand
side of said axis, and
--a set of waveguides formed in said guide plane to guide said
light wave monomodally on command between said input and one or
both of said two outputs, each of said waveguides extending in
linear strip form and having a width at each point and righthand
and lefthand edges in said guide plane, said width constituting a
nominal waveguide width when it is only slightly less than a
multimode waveguide width beyond which said waveguide could guide
said light wave multimodally, each said edge having in each said
area the general shape of a segment of a straight line associated
with said edge, some of said edges being parallel to said
longitudinal direction of the switch and others of said edges being
inclined relative to said longitudinal direction, said set of
waveguides including:
--an input waveguide extending along said axis with a nominal
waveguide width between a rear end constituting said input and a
front end of said waveguide, said front end having an abscissa,
said input waveguide having righthand and lefthand edges, and
--righthand and lefthand output waveguides in optical continuity
with said input waveguides on the righthand side and on the
lefthand side of said axis starting from an output waveguide start
abscissa at least equal to said input waveguide front end abscissa
and extending as far as righthand and lefthand outputs,
respectively, each of said righthand and lefthand output waveguides
including in succession from the rear towards the front:
--a righthand or lefthand receiver segment corresponding to said
output waveguide, extending in said longitudinal direction and
occupying a controlled reception area extending from said output
waveguide start abscissa to a divergence abscissa, said receiver
segment having a width equal to a fraction less than 50% of said
nominal waveguide width, said segment also having a righthand or
lefthand outside edge associated with the same straight line as
said righthand or lefthand edge of the input waveguide, said
segment further having a lefthand or righthand inside edge facing
the inside edge of the receiver segment of the other (i.e. lefthand
or righthand) output waveguide, a gap being left between the inside
edges of said two receiver segments, said gap having a width
constituting a nominal gap width, and
--successive righthand and lefthand divergent segments
corresponding to said righthand or lefthand output waveguide and
receiver segment and extending in succession towards the front and
towards the righthand or lefthand side, respectively, from said
divergence abscissa, each of said righthand or lefthand divergent
segments having a righthand or lefthand outside edge and a lefthand
or righthand inside edge, respectively, said successive divergent
segments of each righthand or lefthand output waveguide
respectively constituting a righthand or lefthand output segment in
an output area situated at abscissae substantially greater than
said divergence abscissa, the straight lines associated with
outside and inside edges of each output segment having the same
inclination constituting a nominal inclination of said output
segment, said straight lines intersecting the straight lines
associated with the outside and inside edges of the corresponding
receiver segment at outside and inside intersection abscissae,
respectively, the width of said output segments being a nominal
width,
--the set of said input and output waveguides constituting for said
light wave a guide structure extending from said rear edge of said
front edge and having a total width in said transverse direction,
said total width constituting firstly said nominal waveguide width
in the areas occupied by said input waveguide and by said output
waveguide receiver segments, and then increasing progressively
towards the front from said divergence abscissa,
--said guide structure having for guiding said light wave a guide
gap extending along said axis and having a width in said transverse
direction, said guide gap being present at those of said abscissae
which are greater than a gap start abscissa not greater than said
output waveguide start abscissa and being absent at those of said
abscissae which are less than said gap start abscissa, a gap width
being defined at each abscissa and being equal to the width of the
guide gap or having a null width depending on whether said guide
gap is present or absent at the abscissa concerned, so that said
gap width is a null width in said input area occupied by said input
guide prior to said gap start abscissa, said gap width being then
equal to said nominal gap width in said controlled reception area,
said gap width finally increasing in said output area to procure
progressive decoupling of two modes of said light wave adapted to
be respectively guided by said output waveguides so that beyond a
front end of said output area said two output waveguides cease to
constitute said guide structure,
--a gap widening rate being defined for each abscissa as the rate
of increase in said gap width as a function of said abscissa so
that said rate is a null rate when said gap width remains at a null
value or remains equal to said nominal gap width and so that said
rate is defined by said nominal inclinations in said output area,
the rate defined by said inclinations constituting a nominal gap
widening rate,
--two transition areas being thus constituted wherein said gap
widening rate is subject to variation, an input transition area
including said gap start abscissa and said output waveguide start
abscissa and subject to increasing and decreasing variations in
said gap widening rate to change said gap width from a value at
first remaining at a null value in said input area to a value
remaining equal to said nominal gap width in said controlled
reception area, an output transition area including said inside
intersection abscissa and changing said gap widening rate from a
value which is initially a null value in said controlled reception
area to a value equal to said nominal gap widening rate in said
output area,
--said switch further including:
--a set of electrodes including a righthand electrode and a
lefthand electrode respectively formed on said righthand output
waveguide and said lefthand output waveguide from their said
receiver segments and at least as far as a rear end of said output
area to enable selective application to said waveguides of
electrical action causing local modification of the refractive
indices of said waveguides,
--said electrodes being responsive to an electrical signal whereby
said local modifications of refractive index optically couple said
input waveguide to said righthand and/or said lefthand output
waveguide, according to the value of said control signal,
--said nominal gap width being chosen to procure mutual electrical
isolation of said righthand and lefthand electrodes in said
controlled reception area,
--in which switch at least one of said two transition areas
constitutes a modified transition area in which said guide gap
modification increases the width of said guide gap, this increase
being localized so as to limit the maximal value of said gap
widening rate in said area.
2. Switch according to claim 1 wherein said gap start abscissa is
substantially less than said input waveguide end abscissa, said
input transition area extending from said gap start abscissa,
including said input waveguide end abscissa and constituting one
said modified transition area,
--said input guide occupying completely said nominal waveguide
width in said area,
--said input transition area including a gap formation area
extending from said gap start abscissa, said input waveguide
dividing in said gap formation area into righthand and lefthand
partial waveguides separated by a median aperture constituting a
first segment of said guide gap and having a width increasing
progressively from zero at said gap start abscissa to said nominal
gap width at a gap stabilization abscissa.
3. Switch according to claim 2 wherein each of said righthand and
lefthand partial waveguides has a respective lefthand or righthand
edge constituting an inside edge of said partial waveguide, said
inside edge being rectilinear and having an inclination between 1
mrd and 10 mrd.
4. Switch according to claim 2 wherein said input transition area
further includes a gap maintenance area extending from said gap
stabilization abscissa to said abscissa at the front end of the
input waveguide and in which said waveguide gap separating said
righthand and lefthand partial waveguides has said nominal gap
width at all points.
5. Switch according to claim 4 wherein said gap maintenance area
has a length between 10% and 150% of the length of said gap
formation area.
6. Switch according to claim 1 wherein said output transition area
extends from an output transition start abscissa to an output
transition end abscissa to constitute one said modified transition
area, one said divergent segment of each of said righthand and
lefthand output waveguides being constituted in this area by a
righthand or lefthand output transition segment having a lefthand
or righthand inside edge facing a righthand or lefthand inside edge
of the other (lefthand or righthand, respectively) of said output
transition segments, said inside edges of said output transition
segments having intermediate inclinations which are fractions of
said nominal inclination.
7. Switch according to claim 6 wherein said output transition start
abscissa is greater than said divergence abscissa.
8. Switch according to claim 6 wherein said inside edge of each of
said output transition segments is rectilinear and has an
inclination between 25% and 65% of said nominal inclination, said
righthand or lefthand output transition segment having a righthand
or lefthand outside edge, respectively, said outside edge being
rectilinear and aligned with said outside edge of the righthand or
lefthand output segment, respectively,
said righthand or lefthand output waveguide further including a
righthand or lefthand outside divergence segment extending in an
outside divergence area from said divergence abscissa to said
output transition start abscissa with a lefthand or righthand
rectilinear inside edge aligned with said inside edge of the
righthand or lefthand receiver segment, respectively, and a
righthand or lefthand rectilinear outside edge aligned with said
outside edge of the righthand or lefthand output segment and the
righthand or lefthand output transition segment, respectively, said
output transition start abscissa being such that the width of each
output waveguide at said abscissa is between 30% and 60% of said
nominal width of said output segment.
9. Switch according to claim 1, said wafer comprising a
monocrystalline semiconductor material in which layers are stacked
in a direction perpendicular to said guide plane, said layers
constituting in succession:
--a highly doped bottom contact layer with a first type of
conductivity in contact with a common electrode,
--a substrate,
--a guide layer having a refractive index higher than said
substrate,
--a confinement layer having a refractive index lower than said
guide layer, said confinement layer having its full thickness in
the area of said waveguides, said layer having a reduced thickness
outside this area and having on top of it a medium having a
refractive index lower than said layer so that said light wave is
guided by said waveguide, and
--a highly doped top contact layer with a second type of
conductivity opposite to said first type, said top contact layer
being present only where said confinement layer has said full
thickness,
--said righthand and lefthand electrodes being present on said top
contact layer in part of the area of said output waveguides,
--said input transition area further including a contact layer
interruption area extending between said input waveguide front end
and output waveguide start abscissae so that absence of said top
contact layer in said contact layer interruption area prevents
unwanted electrical contact between said righthand and lefthand
electrodes through said top contact layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns an optical switch of the kind which
has an optical input and two optical outputs and receives an
electrical control signal. It couples the input to one or other of
the outputs, the coupled output being selected by the electrical
signal.
2. Description of the Prior Art
Switches of this kind are used, for example, in optical
telecommunication networks to constitute switching matrices. It is
desirable for these matrices to have high capacities and to be
compatible with wavelength division multiplexing. An optical switch
for this purpose must have a wide bandwidth, a low sensitivity to
polarization, low insertion losses and, most importantly, a high
extinction rate. The extinction rate is the ratio of the luminous
power at the selected output to that at the other output.
Directional optical couplers are known and have a high extinction
rate. They are sensitive to polarization and have a narrow
bandwidth, however.
Y-branch digital optical switches (DOS) are also known and have a
generic structure described in detail below.
They are insensitive to polarization. Modifications have been made
to them to increase their extinction rate. They are described, for
example, in an article in Journal of Lightwave Technology, Vol. 11,
No. 2, February 1993, pp 379-387, "Reduction of Voltage-length
Product for Y-Branch Digital Optical Switch", H. Okayama and M.
Kawahara.
The extinction rate of these switches can be increased by
increasing the power of the control signal. Any such increase is
costly, however, and can cause problems with the removal of heat,
especially in the case of a high-capacity switching matrix. This is
why the extinction rate of known Y-branch digital optical switches
in practise remains too low for the expressed requirements.
One object of the present invention is to increase the extinction
rate of a Y-branch digital optical switch in a simple manner and
without increasing the power of a control signal. Another object of
the present invention is by this means to provide optical switching
matrices of increased capacity.
SUMMARY OF THE INVENTION
The present invention consists in a Y-branch digital optical switch
in which an input waveguide and two divergent output waveguides
constitute a guide structure having a median gap between said two
output waveguides constituting a guide gap, electrodes controlling
the refractive indices of said two output waveguides to couple said
input waveguide to one or both output waveguides according to the
value of a control signal, in which switch, in at least one
modified transition area, a gap modification renders the variation
of the width of said guide gap more progressive.
The gap modifications can comprise a median aperture at the end of
the input waveguide and/or a progressive variation in the
inclination of two facing edges of the two output waveguides.
How the present invention may be put into effect is described in
more detail hereinafter by way of nonlimiting example and with
reference to the appended diagrammatic drawings. When the same
component is shown in more than one figure it is always identified
by the same reference symbol.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a switch in accordance with the
present invention.
FIG. 2 is a plan view of the guide structure of this switch.
FIG. 3 is a plan view of the guide structure of a prior art
Y-branch digital optical switch.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
There will first be described, with reference to FIGS. 1 and 2,
components of the switch of the invention which have the same
function as corresponding components of the prior art switch, these
components being referred to hereinafter as "common components".
When a common component of the prior art switch is shown in FIG. 3
it is denoted by the letter P followed by the reference symbols
which denote the corresponding common component of the switch of
the invention.
One such common component of the switch of the present invention is
a wafer 2 (see FIG. 1) of a material such as a monocrystalline
semiconductor material, for example, which is transparent to a
light wave to be processed, for example an infrared light wave. A
refractive index of at least one layer of this material is
responsive to electrical action, for example to variation of the
density of charge carriers injected into this layer by an electric
current or removed therefrom by an electric voltage. The wafer lies
in a guide plane 4 which is horizontal, for example. It includes a
vertical succession of horizontal layers, as follows:
--A heavily doped bottom contact layer 20 of a first conductivity
type (for example the n type) in contact with a common electrode
MC.
--An n type (for example) substrate 22.
--A guide layer 24 having a refractive index higher than that of
the substrate.
--A confinement layer 26 having a refractive index lower than that
of the guide layer. The confinement layer has a full thickness in
the area of three optical waveguides constituting an input
waveguide HE and two output waveguides HD and HG. It thickness is
reduced outside this area. Above it is a medium having a refractive
index lower than that of this layer, typically the atmosphere, so
that said light wave is guided by these waveguides.
--Finally, a heavily doped top contact layer 28 with a second
conductivity type opposite the first (i.e. in this example the p
type). The top contact layer 28 is present only where the
confinement layer 26 has its full thickness. Righthand and lefthand
electrodes MD and MG are formed on the top contact layer 28 in the
area of the output waveguides HD and HG.
In the guide plane 4 the wafer 2 has:
--a rear edge CR,
--a front edge CV,
--an axis OX (see FIG. 2) with abscissae x1, . . . , x9 increasing
from the rear edge to the front edge in a longitudinal direction
defined by this axis,
--a righthand side CD,
--a lefthand side CG, and
--a transverse direction DT joining the righthand side and the
lefthand side.
Areas Z1, . . . , Z8 are each defined by one of said abscissae or
between two of said abscissae.
The switch includes:
--an input E on said rear edge to receive said light wave,
--two outputs D, G on said front edge to output said light wave,
these two outputs constituting a righthand output D on the
righthand side of said axis and a lefthand output G on the lefthand
side of said axis, and
--a combination of said waveguides HE, HD, HG. The latter are
formed in the guide plane 4 to guide the wave on command
monomodally between this input and one or both of the two
outputs.
Each of these waveguides is in the form of a linear strip. It has a
width at all points. It also has righthand and lefthand edges in
the guide plane. Its width constitutes a nominal waveguide width W
when it is only slightly less than (for example between 50% of and
100% of) a multimode guide width beyond which the guide could guide
said light wave multimodally. Each edge has in each area the
general shape of a straight line segment associated with that edge.
Some of these edges are parallel to the longitudinal direction of
the switch. Others of these edges are inclined relative to this
direction.
The set of guides includes:
--an input guide HE extending along the axis OX with a nominal
waveguide width between a rear end constituting said input E and a
front end HEV of the guide, this front end having an abscissa x3,
the input waveguide having righthand and lefthand edges HED, HEG,
and
--a righthand output waveguide HD and a lefthand output waveguide
HG extending in optical continuity with the input waveguide to the
righthand side and to the lefthand side of the axis OX from an
output waveguide starting abscissa x4 at least equal to the
abscissa x3 at the front end of the input waveguide as far as the
righthand and lefthand outputs D and G, respectively.
Each righthand (or lefthand) output waveguide includes segments in
sequence from the rear towards the front:
--A righthand receiver segment SD1 (or lefthand receiver segment
SG1) corresponding to this output waveguide and extending in said
longitudinal direction. Each of these receiver segments occupies a
controlled reception area Z5 extending from said output waveguide
start abscissa x4 to a divergence abscissa x5. Its width is less
than 50% of said nominal waveguide width W. It also has a righthand
outside edge SD1D (or lefthand outside edge SG1G) associated with
the same straight line as the righthand edge HED (or the lefthand
edge HEG) of the input waveguide HE. Finally, it has a lefthand
inside edge SD1G (or righthand inside edge SG1D) facing the inside
edge of the receiver segment of the other output waveguide, i.e.
the lefthand (or righthand) output waveguide. A gap 6 is left
between the inside edges of the two receiver segments. This gap has
a constant width constituting a nominal gap width LN.
--Finally, righthand divergence segments SD2, . . . , SD4 or
lefthand divergence segments SG2, . . . , SG4 correspond to said
output waveguides and righthand or lefthand receiver segment. They
extend in sequence towards the front and towards the righthand (or
lefthand) side from said divergence abscissa. Each righthand (or
lefthand SG4) divergence segment has a righthand (or lefthand SG4G)
outside edge and a lefthand (or righthand SG4D) inside edge. The
successive divergent segments of each righthand (or lefthand)
output waveguide constitute a righthand SD4 (or lefthand SG4)
output segment in an output area Z8 situated at abscissae x8, x9
substantially greater than the divergence abscissa x5. The straight
lines associated with the outside and inside edges SD4D, SD4G,
SG4D, SG4G of each output segment have the same inclination
constituting a nominal inclination A of the output segment. They
intersect the straight lines associated with the outside and inside
edges of the corresponding receiver segment at respective outside
and inside intersection abscissae x5, x7. In this example the
outside intersection abscissa is the divergence abscissa x5. The
width of these output segments is the nominal width W.
For the light wave to be treated the set of input and output
waveguides constitutes a guide structure extending from the rear
edge CR to the front edge CV and having a total width WT in the
transverse direction DT. This total width constitutes firstly the
nominal waveguide width W in the areas Z1, . . . , Z5 occupied by
the input waveguide HE and said receiver segments SD1 and SG1 of
the output waveguides. It then increases progressively towards the
front from the divergence abscissa x5.
The guide structure for guiding said light wave has a guide gap 6
extending along the axis OX and having a width in the transverse
direction DT. The guide gap 6 is present at the abscissae x2, . . .
, x9 which are greater than a gap start abscissa x2 which is not
greater than said output waveguide start abscissa x4. The gap is
absent at abscissae x0 less than the gap start abscissa. A gap
width LH is defined at each abscissa. It is equal to the width of
the guide gap or has a null value according to whether the guide
gap is present or absent at the abscissa concerned. The gap width
is therefore a null width in an input area Z1 occupied by the input
waveguide HE prior to the gap start abscissa x2. It is then equal
to the nominal gap width LN in the controlled reception area Z5.
Finally, it increases in the output area Z8 to procure progressive
decoupling of two modes of said light wave which can be guided by
respective output waveguides so that beyond a front end of this
output area the two output waveguides cease to cooperate to
constitute a guide structure.
A gap widening rate can be defined at each abscissa as a rate of
increase of the gap width as a function of the abscissa. This rate
therefore has a null value if the gap width remains at a null value
or remains equal to the nominal gap width. In the output area Z8
this rate is defined by the nominal inclinations and then
constitutes a nominal gap widening rate. Specifically, the
inclinations of the two output waveguides are the same, the two
output waveguides being symmetrical to the OX axis and therefore at
the same angle A to that axis. The nominal gap widening rate
therefore has the value tan2A.
The arrangements as described above define two transition areas,
one at the input and the other at the output, in which the gap
widening rate as defined above is subject to variation:
The input transition area Z2, Z3, Z4 includes gap start and output
waveguide start abscissae x2, x4. In this area increasing and
decreasing variations in the gap widening rate cause the gap width
to change from a value initially remaining at a null value in the
input area Z1 to a value then remaining equal to the nominal gap
width LN in the controlled reception area Z5.
The output transition area Z7 includes the inside intersection
abscissa x7. It causes the gap widening rate to change from a value
which is initially a null value in the vicinity of the controlled
reception area Z5 to a final value equal to the nominal gap
widening rate in the output area Z8.
The righthand electrode MD and the lefthand electrode MG are
respectively formed on the righthand output waveguide HD and the
lefthand output waveguide HG, starting from their receiver segments
SD1 and SG1. They continue at least as far as a rear end of the
output area Z8 and preferably (as shown) as far as a front end x9
of the these areas. They enable selective application to these
waveguides of the electrical action required to cause the required
local modifications of their refractive indices.
The switch finally includes an electrical supply 8, 10, 12
receiving a control signal J and responding thereto by applying to
the electrodes an electrical signal I so that said local
modifications of the refractive indices optically couple the input
waveguide HE to said righthand output waveguide HD and/or to the
lefthand output waveguide HG, depending on the value of the control
signal.
Specifically, in this example, the control signal J controls an
electric switch 12. The latter activates one of the righthand and
lefthand electrodes. It connects the latter to a source 8 of a
positive potential relative to the ground connection constituted by
common electrode MC. A current then flows across the wafer 2 from
the activated electrode and injects charge carriers into the guide
layer 24.
The electrode which is not activated is connected to a negative
potential source 10 to cause depletion of carriers under this
electrode. This reduces the refractive index under the activated
electrode and the light wave to be treated received in the
waveguide HE passes into the output waveguide with the higher
index.
The nominal gap width LN previously referred to is chosen to ensure
mutual electrical isolation of the righthand and lefthand
electrodes MD, MG in the controlled reception area Z5.
In the prior art switch the input waveguide PHE and the output
waveguides PHD and PHG have the shapes shown in FIG. 3.
In accordance with the present invention at least one of said two
transition areas is subject to a modification whereby the guide gap
width is locally modified, as compared with what has previously
been described, to limit the maximal value of the gap widening rate
in this area. Such modification can advantageously take either or
preferably both of two forms, depending on whether one or both
transition areas are modified. These two forms are shown in FIGS. 1
and 2.
In a first form of modification the gap start abscissa x1 is
substantially less than the abscissa at the end of the input
waveguide x3. The input transition area Z2, Z3, Z4 then extends
from this gap start abscissa x1, including this input waveguide end
abscissa x3 and constituting a modified transition area. The input
waveguide occupies all of its nominal width W in the input area Z1.
The input transition area includes a gap formation area Z2
extending from said gap start abscissa x1. In this gap formation
area the input waveguide HE divides into righthand and lefthand
partial waveguides SED and SEG separated by a median aperture
constituting a first segment of the guide gap 6. The width of this
opening increases progressively from a null width at the gap start
abscissa x1 to the nominal gap width LN at a gap stabilization
abscissa x2.
Each of the righthand SED and lefthand SEG partial waveguides then
has a respective lefthand edge SEDG or righthand edge SEGD
constituting an inside edge of this partial waveguide.
Specifically, this inside edge is rectilinear and has an
inclination which is less than said nominal inclination A of the
output segments SD4 and SG4. This inclination is typically between
one milliradian and ten milliradians.
The input transition area Z2, Z3, Z4 preferably further includes a
gap maintenance area Z3 extending from the gap stabilization
abscissa x2 to the abscissa at the front end of the input waveguide
x3. In this gap maintenance area the aperture between the righthand
and lefthand partial waveguides SED, SEG has said nominal gap width
LN at all points.
Specifically, the gap maintenance area Z3 has a length between 10%
and 150% of the length of the gap formation area Z2.
This first form of modification reduces optical losses when light
passes from the input waveguide to whichever of the output
waveguides is to receive the light wave to be treated. This
reduction results from the fact that the optical mode guided by the
end of the input waveguide is better suited to the shape of the
mode that will be guided by one of the output waveguides.
In a second form of modification the output transition area Z7
extends from an output transition start abscissa x6 to an output
transition end abscissa x8 to constitute a modified transition
area. In this area a divergent segment of each righthand or
lefthand output waveguide is constituted by a righthand SD3 (or
lefthand SG3) output transition segment having a lefthand SD3G (or
lefthand SG3D) inside edge facing a righthand (or lefthand) inside
edge of the other output transition segment (i.e. the lefthand (or
righthand) segment), and these inside edges of the output
transition segments have intermediate inclinations which are a
fraction of the nominal inclination A, for example A/2. The output
transition start abscissa x6 is preferably greater than the
divergence abscissa x5.
Specifically, the inside edge SG3D of each output transition
segment SG3 is rectilinear and has an inclination between 25% and
75% and preferably equal to around 50% of the nominal inclination
A. This righthand SD3 or lefthand SG3 output transition segment has
a righthand SD3D (or lefthand SG3G) outside edge. This outside edge
is rectilinear and aligned with the outside edge SD4D, SG4G of the
righthand SD4 (or lefthand SG4) output segment. The righthand HD
(or lefthand HG) output waveguide preferably further includes a
righthand SD2 (or lefthand SG2) outside divergence segment in an
outside divergence area Z6 extending from the divergence abscissa
x5 to the output transition start abscissa x6. This outside
divergence segment has a lefthand SD2G (or righthand SG2D)
rectilinear inside edge aligned with the inside edge SD1G, SD1D of
the righthand SD1 (or lefthand SG1) receiver segment and a
righthand SD2D (or lefthand SG2G) rectilinear outside edge aligned
with the outside edge SD4D, SG4G, SD3D, SG3G of the righthand SD4
(or lefthand SG4) output segment and the righthand SD3 (or lefthand
SG3) output transition segment. The output transition start
abscissa x6 is preferably such that the width of each output
waveguide at this abscissa is between 30% and 70% and preferably
equal to around 50% of the nominal width W of the output
segment.
This second form of modification further reduces optical losses of
the switch, in particular in the fully switched state, i.e. when
all the light from the light wave to be treated is output from a
single one of two outputs.
In a manner that is known in itself, the input transition area Z2,
Z3, Z4 preferably further includes a contact layer interruption
area Z4 extending between the input waveguide front end abscissa x3
and the output waveguide start abscissa x4. Interrupting the top
contact layer 28 prevents unwanted electrical contact between the
righthand and lefthand electrodes MD, MG through this layer.
A method that is known in itself to facilitate fabrication of the
switch using auto-alignment methods reduces the thickness of the
confinement layer 26 in the contact layer interruption area. This
constitutes a spurious interruption of the optical waveguide
structure. However, the length of the interruption is sufficiently
small for optical functioning of the switch not be significantly
affected.
The switch of the invention described by way of example is
symmetrical about its axis OX. An asymmetric switch can have one
output waveguide wider than the other, especially if the switch is
dedicated to fail-safe applications requiring no electrical
consumption in the passive state, i.e. in the absence of any
control signal, with all of the light directed to a single output.
It is to be understood that the invention is equally advantageous
when applied to any such asymmetric switch.
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